In a steam turbine facility, steam is led from a boiler to a turbine casing via a pipe so as to drive the turbine by the steam. Further, a steam valve (a stop valve, a governing valve, etc) is provided in a steam pipe for supplying the steam to the turbine casing. The steam valve is configured to cut off the steam entering the turbine casing and to adjust the steam flow.
In a conventional steam turbine facility, a graphite type spiral-wound gasket was mainly used for a seal portion of a flange connection part of the pipes, the steam valve, and the like. The graphite type spiral-wound gasket, herein, indicates a semi-metallic gasket formed by alternately winding a metallic hoop and a filler of graphite-type material into a spiral shape.
For instance, described in Patent Literature 1 is a spiral gasket in which an expanded graphite tape containing inorganic oxidation inhibitor is used as filler to improve the maximum temperature of use in oxide atmosphere up to approximately 500° C.-600° C.
From the perspective of improving performance of steam turbine facilities, the temperature of the steam has been increased. A steam turbine facility where steam temperature is 650° C. or higher is becoming popular. In this type of steam turbine facility used under harsh steam-temperature conditions, when the graphite type spiral-wound gasket described in Patent Literature 1 is used, there is a concern about deterioration of sealability of the gasket due to loss of graphite by oxidation.
Described in Patent Literature 2 is a self-sealing gasket which can be used in the steam turbine facility under the harsh steam-temperature conditions (see paragraph 0015, Patent Literature 2). The cross-sectional shape of this self-sealing gasket is a circular ring with a slit. This self-sealing gasket seals a gap between a steam valve casing and a bonnet by being held between the steam valve casing and the bonnet and making line contact with them. When using the self-sealing gasket, steam passes through the slit and is guided into the gasket to apply an internal pressure of the steam to the gasket. As a result, even if a fastening force of the casing and the bonnet is small, it is possible to achieve excellent sealability with respect to high temperature and high pressure steam.